1. Field of the Invention
The present invention relates generally to packing machines which employ a vacuum to pack low density powder materials into valve bags and provide an accurate amount of such material based on the weight of the filled bag.
2. Background Art
The original vacuum packer designed especially for packing light density powders into valve bags was invented in the late 1950's by Clarence Carter. That machine comprises a basic framework. This framework supports a weighing scale mechanism. Suspended from the weighing mechanism is a large vacuum chamber with an access door on one side of the chamber. This chamber is suspended from the scale mechanism and checked back to the frame with check rods to keep the chamber from swinging laterally and longitudinally.
Mounted within the chamber and to the chamber is the bag filling spout, bag support chair, interior side panels, vacuum and atmosphere piping connections and a clean-out port. All of these devices are attached to the chamber and therefore suspended from the weigh scale mechanism.
To operate the machine, an operator would place an empty valve bag onto the filling spout of the machine. The chamber door would close and the chamber would be evacuated of air by a vacuum pump.
The product feed valve would then be opened, thereby admitting product into the valve bag by drawing air through the pores of the bag and pulling the product into the bag.
Upon reaching the desired set weight, the machine would stop the flow of product into the bag. Then, after a preset time, the chamber doors would open and the bag would be removed from the filling machine.
The original design had several problems that have proven very difficult to overcome. The problems were as follows:
The fact that the entire chamber, side plates, interior retention panels, and other mechanical devices are suspended from the scale mechanism requires a scale capable of supporting approximately 500 pounds of tare weight, while attempting to accurately weigh a bag to a weight of typically 10 to 20 pounds. Therefore the resolution of the scale mechanism has to be set to operate at 500 pounds.
The fact that the entire chamber with the related control devices is suspended from the scale mechanism requires several flexible connections between the main framework of the machine and the scale supported chamber. These connections include the connection from the product feed tube to the filling spout, the connection for the vacuum and atmospheric valve piping, and the connection for the lower chamber clean-out valves. Each of the connections requires extremely careful alignment between the chamber and the main framework.
In addition, the chamber itself has a tendency to distort when under vacuum. This distortion greatly affects the scale readout. This is especially true when using a single load-cell type of scale detection, as these devices are extremely sensitive to any distortion of the live load.
The combination of these factors produces a difference in the scale reading of up to three pounds of the scale supported chamber weight when under vacuum conditions and when under atmospheric conditions.
The difference in reading makes it extremely difficult to maintain consistent, accurate weights of the filled bags. In an effort to overcome these problems, the machine operators continuously manipulate the final cut-off point of the scale system to compensate for errors caused by the above outlined conditions.
As a result of the above problems, a new and novel approach is required to the arrangement of the chamber and scale or weighing system as applied to vacuum packers.